1. Field of the Invention
The invention concerns a method of detecting false locks between a reference signal and a signal to be demodulated by coherent digital demodulation and a device implementing a method of this kind.
2. Description of the Prior Art
A first conventional type microwave receiving system comprises in series after the receive antenna a microwave band filter, a low-noise amplifier and a mixer carrying out transposition to an intermediate frequency by means of a local oscillator. A system of this kind then comprises an intermediate frequency preamplifier, circuits for correcting group delay time and an intermediate frequency amplifier. The intermediate frequency demodulator circuit then restores the baseband signal, the encoded signal being recovered after processing the baseband signal. In receiving systems of this kind the double transposition from the microwave band to the intermediate frequency and then from the intermediate frequency to the baseband entails the use of a set of intermediate frequency circuits, in particular a local oscillator.
A second type of microwave receiving system enables direct demodulation of a received microwave signal with a reduced number of components, the intermediate frequency circuits being eliminated. U.S. Pat. No. 4,559,499 describes a microwave direct demodulation device for demodulating a modulated signal produced by mixing two carriers in phase quadrature each modulated by digital signals; it comprises a modulator circuit the input of which is connected to a received signal input associated with an oscillator operating at the carrier frequency of the received signal and controlled so that the phase of the carrier from the oscillator coincides with the phase of the received signal, the demodulator circuit comprising a separator connected to first inputs of two symmetrical mixers, a coupler introducing a phase shift of 90.degree. into one channel and the outputs of which are connected to second inputs of second mixers, and low-pass filters connected to the outputs of the mixers delivering the demodulated digital signals.
In any coherent demodulation system it is necessary to have a phase reference, obtained by recovering the carrier: the needed information is contained in the phase of this carrier.
Whether demodulation is direct or involves transposition to an intermediate frequency, there occur "false locks" between the reference signal and the signal to be demodulated, resulting in erroneous phase information that cannot normally be corrected. In the case of modulation with four phase states, for example, these false locks are situated at multiples of one quarter the baud rate on either side of the center frequency.
The lower the transmit baud rate the nearer these false locks are to the carrier frequency, so that the stability of the VCO (voltage-controlled oscillator), especially its temperature stability, must be relatively high and therefore possibly difficult to achieve, especially if the baud rate is low and the carrier frequency is high.
If the stability of the VCO (dF/FO) is in the same order of magnitude as the difference between the frequency of the false locks and the carrier frequency FO, phase comparators cannot distinguish true locks from false locks in the search phase (frequency search that must be better than the VGO drift to enable locking).
An object of the invention is to provide a solution to this problem.